Small electrode for a chalcogenide switching device and method for fabricating same

ABSTRACT

A memory cell and a method of fabricating the memory cell having a small active area. By forming a spacer in a window that is sized at the photolithographic limit, a pore may be formed in dielectric layer which is smaller than the photolithographic limit. Electrode material is deposited into the pore, and a layer of structure changing material, such as chalcogenide, is deposited onto the lower electrode, thus creating a memory element having an extremely small and reproducible active area.

This application is a Divisional of application Ser. No. 08/854,220filed May 09, 1997.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

The present invention relates generally to semiconductor fabricationtechniques and, more particularly, to a method for fabricating smallelectrodes for use with a chalcogenide switching device, such as, forexample, a chalcogenide memory cell.

2. Background of the Related Art

Microprocessor-controlled integrated circuits are used in a wide varietyof applications. Such applications include personal computers, vehiclecontrol systems, telephone networks, and a host of consumer products. Asis well known, microprocessors are essentially generic devices thatperform specific functions under the control of a software program. Thisprogram is stored in a memory device coupled to the microprocessor. Notonly does the microprocessor access a memory device to retrieve theprogram instructions, it also stores and retrieves data created duringexecution of the program in one or more memory devices.

There are a variety of different memory devices available for use inmicroprocessor-based systems. The type of memory device chosen for aspecific function within a microprocessor-based system depends largelyupon what features of the memory are best suited to perform theparticular function. For instance, volatile memories, such as dynamicrandom access memories (DRAMs), must be continually powered in order toretain their contents, but they tend to provide greater storagecapability and programming options and cycles than non-volatilememories, such as read only memories (ROMs). While non-volatile memoriesthat permit limited reprogramming exist, such as electrically erasableand programmable “ROMs,” all true random access memories, i.e., thosememories capable of 10¹⁴ programming cycles are more, are volatilememories. Although one time programmable read only memories andmoderately reprogrammable memories serve many useful applications, atrue nonvolatile random access memory (NVRAM) would be needed to surpassvolatile memories in usefulness.

Efforts have been underway to create a commercially viable memorydevice, which is both random access and nonvolatile, using structurechanging memory elements, as opposed to charge storage memory elementsused in most commercial memory devices. The use of electrically writableand erasable phase change materials, i.e., materials which can beelectrically switched between generally amorphous and generallycrystalline states or between different resistive states while incrystalline form, in memory applications is known in the art and isdisclosed, for example, in U.S. Pat. No. 5,296,716 to Ovshinsky et al.,the disclosure of which is incorporated herein by reference. TheOvshinsky patent is believed to indicate the general state of the artand to contain a discussion of the general theory of operation ofchalcogenide materials, which are a particular type of structurechanging material.

As disclosed in the Ovshinsky patent, such phase change materials can beelectrically switched between a first structural state, in which thematerial is generally amorphous, and a second structural state, in whichthe material has a generally crystalline local order. The material mayalso be electrically switched between different detectable states oflocal order across the entire spectrum between the completely amorphousand the completely crystalline states. In other words, the switching ofsuch materials is not required to take place in a binary fashion betweencompletely amorphous and completely crystalline states. Rather, thematerial can be switched in incremental steps reflecting changes oflocal order to provide a “gray scale” represented by a multiplicity ofconditions of local order spanning the spectrum from the completelyamorphous state to the completely crystalline state.

These memory elements are monolithic, homogeneous, and formed ofchalcogenide material typically selected from the group of Te, Se, Sb,Ni, and Ge. This chalcogenide material exhibits different electricalcharacteristics depending upon its state. For instance, in its amorphousstate the material exhibits a higher resistivity than it does in itscrystalline state. Such chalcogenide materials can be switched betweennumerous electrically detectable conditions of varying resistivity innanosecond time periods with the input of picojoules of energy. Theresulting memory element is truly non-volatile. It will maintain theintegrity of the information stored by the memory cell without the needfor periodic refresh signals, and the data integrity of the informationstored by these memory cells is not lost when power is removed from thedevice. The memory material is also directly overwritable so that thememory cells need not be erased, i.e., set to a specified startingpoint, in order to change information stored within the memory cells.Finally, the large dynamic range offered by the memory materialtheoretically provides for the gray scale storage of multiple bits ofbinary information in a single cell by mimicking the binary encodedinformation in analog form and, thereby, storing multiple bits of binaryencoded information as a single resistance value in a single cell.

The operation of chalcogenide memory cells requires that a region of thechalcogenide memory material, called the “active region,” be subjectedto a current pulse to change the crystalline state of the chalcogenidematerial within the active region. Typically, a current density ofbetween about 10⁵ and 10⁷ amperes/cm² is needed. To obtain this currentdensity in a commercially viable device having at least 64 millionmemory cells, for instance, the active region of each memory cell mustbe made as small as possible to minimize the total current drawn by thememory device. Currently, chalcogenide memory cells are fabricated byfirst creating a diode in a semiconductor substrate. A lower electrodeis created over the diode, and a layer of dielectric material isdeposited onto the lower electrode. A small opening is created in thedielectric layer. A second dielectric layer, typically of siliconnitride, is then deposited onto the dielectric layer and into theopening. The second dielectric layer is typically about 40 Angstromsthick. The chalcogenide material is then deposited over the seconddielectric material and into the opening. An upper electrode material isthen deposited over the chalcogenide material.

A conductive path is then provided from the chalcogenide material to thelower electrode material by forming a pore in the second dielectriclayer by a process known as “popping.” Popping involves passing aninitial high current pulse through the structure to cause the seconddielectric layer to breakdown. This dielectric breakdown produces aconductive path through the memory cell. Unfortunately, electricallypopping the thin silicon nitride layer is not desirable for a highdensity memory product due to the high current and the large amount oftesting time required. Furthermore, this technique may produce memorycells with differing operational characteristics, because the amount ofdielectric breakdown may vary from cell to cell.

The active regions of the chalcogenide memory material within the poresof the dielectric material created by the popping technique are believedto change crystalline structure in response to applied voltage pulses ofa wide range of magnitudes and pulse durations. These changes incrystalline structure alter the bulk resistance of the chalcogenideactive region. Factors such as pore dimensions (e.g., diameter,thickness, and volume), chalcogenide composition, signal pulse duration,and signal pulse waveform shape may affect the magnitude of the dynamicrange of resistances, the absolute endpoint resistances of the dynamicrange, and the voltages required to set the memory cells at theseresistances. For example, relatively thick chalcogenide films, e.g.,about 4000 Angstroms, result in higher programming voltage requirements,e.g., about 15-25 volts, while relatively thin chalcogenide layers,e.g., about 500 Angstroms, result in lower programming voltagerequirements, e.g., about 1-7 volts. Thus, to reduce the requiredprogramming voltage, it has been suggested that the cross-sectional areaof the pore should be reduced to reduce the size of the chalcogenideelement.

The energy input required to adjust the crystalline state of thechalcogenide active region of the memory cell is directly proportionalto the minimum lateral dimension of the pore. In other words,programming energy decreases as the pore size decreases. Conventionalchalcogenide memory cell fabrication techniques provide a minimumlateral pore dimension, e.g., the diameter or width of the pore, that islimited by the photolithographic size limit. This results in pore sizeshaving minimum lateral dimensions down to approximately 1 micron.

The present invention is directed to overcoming, or at least reducingthe affects of, one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a memory cell. The memory cell includes an access device thatis formed on a semiconductor substrate. A layer of dielectric materialis disposed on the access device. The layer of dielectric material has aport therein. The pore is smaller that the photolithographic limit. Afirst layer of conductive material is disposed within the pore to form afirst electrode. A layer of structure changing material is disposed onthe first electrode. A second layer of conductive material is disposedon the layer of structure changing material to form a second electrode.

In accordance with another aspect of the present invention, there isprovided a memory array. The memory array includes a plurality of memorycells. Each memory cell includes an access device that is formed on asemiconductor substrate. A layer of dielectric material is disposed onthe access device. The layer of dielectric material has a pore therein.The pore is smaller than the photolithographic limit. A first layer ofconductive material is disposed within the pore to form a firstelectrode. A layer of structure changing material is disposed on thefirst electrode. A second layer of conductive material is disposed onthe layer of structure changing material to form a second electrode. Thememory array also includes a grid that is coupled to the plurality ofmemory cells. The grid is formed by a first plurality of conductivelines that generally extend in a first direction and a second pluralityof conductive lines that generally extend in a second direction.

In accordance with still another aspect of the present invention, thereis provided a method of fabricating a memory cell. The method includesthe steps: (a) forming an access device on a semiconductor substrate;(b) depositing a layer of dielectric material on the access device; (c)forming a pore in the layer of dielectric material, where the pore issmaller than the photolithographic limit; (d) depositing a first layerof conductive material within the pore to form a first electrode; (e)depositing a layer of structure changing material on the firstelectrode; and (f) depositing a second layer of conductive material onthe layer of structure changing material to form a second electrode.

In accordance with yet another aspect of the present invention, there isprovided a method of fabricating a memory array. The method includes thesteps of (a) forming an access device on a semiconductor substrate; (b)forming a first plurality of conductive lines, where each of the firstplurality of conductive lines is coupled to respective access devices;(c) depositing a layer of dielectric material on the access device; (d)forming a pore in the layer of dielectric material, where the pore issmaller than the photolithographic limit; (e) depositing a first layerof conductive material within the pore to form a first electrode; (f)depositing a layer of structure changing material on the firstelectrode; (g) depositing a second layer of conductive material on thelayer of structure changing material to form a second electrode; and (h)forming a second plurality of conductive lines, where each of the secondplurality of conductive lines is coupled to respective secondelectrodes.

In accordance with a further aspect of the present invention, there isprovided a method of fabricating an array of pores. The method includesthe steps of (a) forming a mask over a layer of dielectric material,where the mask has a plurality of windows therein exposing portions ofthe layer of dielectric material, and where the windows are sized at thephotolithographic limit; (b) forming a spacer within each of thewindows, where each spacer covers a peripheral portion of the respectiveexposed portion of the layer of dielectric material to create a secondwindow that exposes a portion of the layer of dielectric materialsmaller than the photolithographic limit; and (c) removing the exposedportions of the layer of dielectric material created by the secondwindows to create the pores.

In accordance with an even further aspect of the present invention,there is provided a memory cell. The memory cell includes an accessdevice that is formed on a semiconductor substrate. A layer ofdielectric material is disposed on the access device. The layer ofdielectric material has a pore therein. The pore is formed by forming amask over the layer of dielectric material. The mask has a windowtherein which exposes a portion of the layer of dielectric material. Thewindow is sized at the photolithographic limit. A spacer is formedwithin the window. The spacer covers a peripheral portion of the exposedportion of the layer of dielectric material to create a second windowexposing a portion of the layer of dielectric material smaller than thephotolithographic limit. The exposed portion of the layer of dielectricmaterial created by the second window is removed to create the pore. Afirst layer of conductive material is disposed within the pore to form afirst electrode. A layer of structure changing material is disposed onthe first electrode. A second layer of conductive material is disposedon the layer of structure changing material to form a second electrode.

DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 illustrates a schematic depiction of a substrate containing amemory device which includes a memory matrix and peripheral circuitry;

FIG. 2 illustrates an exemplary schematic depiction of the memory matrixor array of FIG. 1;

FIG. 3 illustrates an exemplary memory cell having a memory element,such as a resistor, and an access device, such as a diode;

FIG. 4 illustrates a top view of a portion of a semiconductor memoryarray;

FIG. 5 illustrates a cross-sectional view of an exemplary memory cell atan early stage of fabrication;

FIG. 6, FIG. 7, and FIG. 8 illustrate the formation of a spacer and asmall pore for the exemplary memory element;

FIG. 9 illustrates the small pore of the memory element;

FIG. 10 and FIG. 11 illustrate the formation of an electrode in thesmall pore;

FIG. 12 illustrates the deposition of memory material over the lowerelectrode;

FIG. 13 illustrates the deposition of the upper electrode of the memorycell;

FIG. 14 illustrates the deposition of an insulative layer and an oxidelayer over the upper electrode of the memory cell; and

FIG. 15 illustrates the formation of a contact extending through theoxide and insulative layer to contact the upper electrode.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Turning now to the drawings, and referring initially to FIG. 1, a memorydevice is illustrated and generally designated by a reference numeral10. The memory device 10 is an integrated circuit memory that isadvantageously formed on a semiconductor substrate 12. The memory device10 includes a memory matrix or array 14 that includes a plurality ofmemory cells for storing data, as described below. The memory matrix 14is coupled to periphery circuitry 16 by the plurality of control lines18. The periphery circuitry 16 may include circuitry for addressing thememory cells contained within the memory matrix 14, along with circuitryfor storing data in and retrieving data from the memory cells. Theperiphery circuitry 16 may also include other circuitry used forcontrolling or otherwise insuring the proper functioning of the memorydevice 10.

A more detailed depiction of the memory matrix 14 is illustrated in FIG.2. As can be seen, the memory matrix 14 includes a plurality of memorycells 20 that are arranged in generally perpendicular rows and columns.The memory cells 20 in each row are coupled together by a respectiveword line 22, and the memory cells 20 in each column are coupledtogether by a respective digit line 24. Specifically, each memory cell20 includes a word line node 26 that is coupled to a respective wordline 22, and each memory cell 20 includes a digit line node 28 that iscoupled to a respective digit line 24. The conductive word lines 22 anddigit lines 24 are collectively referred to as address lines. Theseaddress lines are electrically coupled to the periphery circuitry 16 sothat each of the memory cells 20 can be accessed for the storage andretrieval of information.

FIG. 3 illustrates an exemplary memory cell 20 that may be used in thememory matrix 14. The memory cell 20 includes a memory element 30 whichis coupled to an access device 32. In this embodiment, the memoryelement 30 is illustrated as a programmable resistive element, and theaccess device 32 is illustrated as a diode. Advantageously, theprogrammable resistive element may be made of a chalcogenide material,as will be more fully explained below. Also, the diode 32 may be aconventional diode, a zener diode, or an avalanche diode, depending uponwhether the diode array of the memory matrix 14 is operated in a forwardbiased mode or a reverse biased mode. As illustrated in FIG. 3, thememory element 30 is coupled to a word line 22, and the access device 32is coupled to a digit line 24. However, it should be understood thatconnections of the memory element 20 may be reversed without adverselyaffecting the operation of the memory matrix 14.

As mentioned previously, a chalcogenide resistor may be used as thememory element 30. A chalcogenide resistor is a structure changingmemory element because its molecular order may be changed between anamorphous state and a crystalline state by the application of electricalcurrent. In other words, a chalcogenide resistor is made of a statechangeable material that can be switched from one detectable state toanother detectable state or states. In state changeable materials, thedetectable states may differ in their morphology, surface typography,relative degree of order, relative degree of disorder, electricalproperties, optical properties, or combinations of one or more of theseproperties. The state of a state changeable material may be detected bymeasuring the electrical conductivity, electrical resistivity, opticaltransmissivity, optical absorption, optical refraction, opticalreflectivity, or a combination of these properties. In the case of achalcogenide resistor specifically, it may be switched between differentstructural states of local order across the entire spectrum between thecompletely amorphous state and the completely crystalline state.

The previously mentioned Ovshinsky patent contains a graphicalrepresentation of the resistance of an exemplary chalcogenide resistoras a function of voltage applied across the resistor. It is not unusualfor a chalcogenide resistor to demonstrate a wide dynamic range ofattainable resistance values of about two orders of magnitude. When thechalcogenide resistor is in its amorphous state, its resistance isrelatively high. As the chalcogenide resistor changes to its crystallinestate, its resistance decreases.

As discussed in the Ovshinsky patent, low voltages do not alter thestructure of a chalcogenide resistor, while higher voltages may alterits structure. Thus, to “program” a chalcogenide resistor, i.e., toplace the chalcogenide resistor in a selected physical or resistivestate, a selected voltage in the range of higher voltages is appliedacross the chalcogenide resistor, i.e., between the word line 22 and thedigit line 24. Once the state of the chalcogenide resistor has been setby the appropriate programming voltage, the state does not change untilanother programming voltage is applied to the chalcogenide resistor.Therefore, once the chalcogenide resistor has been programmed, a lowvoltage may be applied to the chalcogenide resistor, i.e., between theword line 22 and the digit line 24, to determine its resistance withoutchanging its physical state. As mentioned previously, the addressing,programming, and reading of the memory elements 20 and, thus, theapplication of particular voltages across the word lines 22 and digitlines 24, is facilitated by the periphery circuitry 16.

The memory cell 20, as illustrated in FIG. 3, may offer significantpackaging advantages as compared with memory cells used in traditionalrandom access and read only memories. This advantage stems from the factthat the memory cell 20 is a vertically integrated device. In otherwords, the memory element 30 may be fabricated on top of the accessdevice 32. Therefore, using the memory cell 20, it may be possible tofabricate an X-point cell that is the same size as the crossing area ofthe word line 22 and the digit line 24, as illustrated in FIG. 4.However, the size of the access device 32 typically limits the area ofthe memory cell 20, because the access device 32 must be large enough tohandle the programming current needed by the memory element 30.

As discussed previously, to reduce the required programing current, manyefforts have been made to reduce the pore size of the chalcogenidematerial that forms the memory element 30. These efforts have been madein view of the theory that only a small portion of the chalcogenidematerial, referred to as the “active region,” is structurally altered bythe programming current. However, it is believed that the size of theactive area of the chalcogenide memory element 30 may be reduced byreducing the size of an electrode which borders the chalcogenidematerial. By reducing the active area and, thus, the requiredprogramming current, the size of the access device may be reduced tocreate an X-point cell memory. For example, a cell with a chalcogenidecross-sectional area equivalent to a circle with an 0.2 μm diametermight require a current pulse of 2 mA to program to high resistancestate. If the diameter of the cell is reduced to 0.1 μm the currentcould be reduced to about 0.5 mA. Over certain ranges of operation theprogramming current is directly proportional to the area of the cell.

The actual structure of an exemplary memory cell 20 is illustrated inFIG. 15, while a method for fabricating the memory cell 20 is describedwith reference to FIGS. 5-15. It should be understood that while thefabrication of only a single memory cell 20 is discussed below,thousands of similar memory cells may be fabricated simultaneously.Although not illustrated, each memory cell is electrically isolated fromother memory cells in the array in any suitable manner, such as by theaddition imbedded field oxide regions between each memory cell.

In the interest of clarity, the reference numerals designating the moregeneral structures described in reference to FIGS. 1-4 will be used todescribe the more detailed structures illustrated in FIGS. 5-15, whereappropriate. Referring first to FIG. 5, the digit lines 24 are formed inor on a substrate 12. As illustrated in FIG. 5, the digit line 24 isformed in the P-type substrate 12 as a heavily doped N+type trench. Thistrench may be strapped with appropriate materials to enhance itsconductivity. The access device 32 is formed on top of the digit line24. The illustrated access device 32 is a diode formed by a layer of Ndoped polysilicon 40 and a layer of P+ doped polysilicon 42. Next, alayer of insulative or dielectric material 44 is disposed on top of theP+ layer 42. The layer 44 may be formed from any suitable insulative ordielectric material, such as plasma enhanced CVD SiO_(z), or PECVDsilicon nitride or standard thermal CVD Sa₃Ny.

The formation of a small pore in the dielectric layer 44 is illustratedwith reference to FIGS. 5-9. First, a hard mask 46 is deposited on topof the dielectric layer 44 and patterned to form a window 48, asillustrated in FIG. 6. The window 48 in the hard mask 46 isadvantageously as small as possible. For instance, the window 48 may beformed at the photolithographic limit by conventional photolithographictechniques. The photolithographic limit, i.e., the smallest feature thatcan be patterned using photolithographic techniques, is currently about0.2 μm. Once the window 48 has been formed in the hard mask 46, a layerof spacer material 50 is deposited over the hard mask 46 in a conformalfashion so that the upper surface of the spacer material 50 is recessedwhere the spacer material 50 covers the window 48. Although any suitablematerial may be used for the spacer material 50, a dielectric material,such CVD amorphous or polycrystalline silicon, may be advantageous.

The layer of spacer material 50 is subjected to an anisotropic etchusing a suitable etchant, such as HBr+Cl₂. The rate and time of the etchare controlled so that the layer of spacer material 50 is substantiallyremoved from the upper surface of the hard mask 48 and from a portion ofthe upper surface of the dielectric layer 44 within the window 48,leaving sidewall spacers 52 within the window 48. The sidewall spacers52 remain after a properly controlled etch because the verticaldimension of the spacer material 50 near the sidewalls of the window 48is approximately twice as great as the vertical dimension of the spacermaterial 50 on the surface of the hard mask 46 and in the recessed areaof the window 48.

Once the spacers 52 have been formed, an etchant is applied to thestructure to form a pore 54 in the dielectric layer 44, as illustratedin FIG. 8. The etchant is an anisotropic etchant that selectivelyremoves the material of the dielectric layer 44 bounded by the spacers52 until the P+ layer 42 is reached. As a result of the fabricationmethod to this point, if the window 48 is at the photolithographiclimit, the pore 54 is smaller than the photolithigraphic limit, e.g., onthe order of 0.1 μm. After the pore 54 has been formed, the hard mask 46and the spacers 52 may be removed, as illustrated in FIG. 9. The hardmask 46 and the spacers 52 may be removed by any suitable method, suchas by etching or by chemical mechanical planarization (CMP).

The pore 54 is then filled to a desired level with a material suitableto form the lower electrode of the chalcogenide memory element 30. Asillustrated in FIG. 10, a layer of electrode material 56 is depositedusing collimated physical vapor deposition (PVD). By using collimatedPVD, or another suitable directional deposition technique, the layer ofelectrode material 56 is formed on top of the dielectric layer 44 andwithin the pore 54 with substantially no sidewalls.

Thus, the layer of electrode material 56 on top of the dielectric layer44 may be removed, using CMP for instance, to leave the electrode 56 atthe bottom of the pore 54, as illustrated in FIG. 11. It should beunderstood that the electrode material 56 may be comprised of one ormore materials, and it may be formed in one or more layers. Forinstance, a lower layer of carbon may be used as a barrier layer toprevent unwanted migration between the subsequently depositedchalcogenide material and the P+ type layer 42. A layer of titaniumnitride (TiN) may then be deposited upon the layer of carbon to completethe formation of the electrode 56.

After the lower electrode 56 has been formed, a layer of chalcogenidematerial 58 may be deposited so that it contacts the lower electrode 56,as illustrated in FIG. 12. Various types of chalcogenide materials maybe used to form the chalcogenide memory element 30. For example,chalcogenide alloys may be formed from tellurium, antimony, germanium,selenium, bismuth, lead, strontium, arsenic, sulfur, silicon,phosphorous, and oxygen. Advantageously, the particular alloy selectedshould be capable of assuming at least two generally stable states inresponse to a stimulus, for a binary memory, and capable of assumingmultiple generally stable states in response to a stimulus, for a higherorder memory. Generally speaking, the stimulus will be an electricalsignal, and the multiple states will be different states ofcrystallinity having varying levels of electrical resistance. Alloysthat may be particularly advantageous include tellurium, antimony, andgermanium having approximately 55 to 85 percent tellurium and 15 to 25percent germanium, such as Te₅₆Ge₂₂Sb₂₂.

If the lower electrode 56 is recessed within the pore 54, a portion ofthe chalcogenide material 58 will fill the remaining portion of the pore54. In this case, any chalcogenide material 58 adjacent the pore 54 onthe surface of the dielectric layer 44 may be removed, using CMP forinstance, to create a chalcogenide element of extremely smallproportions. Alternatively, if the lower electrode 56 completely fillsthe pore 54, the chalcogenide material 58 adjacent the pore 54 mayremain, because the extremely small size of the lower electrode 56 stillcreates a relatively small active area in a vertical direction throughthe chalcogenide material 58. Because of this characteristic, even ifthe lower electrode 56 only partially fills the pore 54, as illustrated,the excess chalcogenide material 58 adjacent the pore 54 need not beremoved to create a memory element 30 having an extremely small activearea.

Regardless of which alternative is chosen, the upper electrode 60 isdeposited on top of the chalcogenide material 58, as illustrated in FIG.13. After the upper electrode 60, the chalcogenide material 58, thedielectric layer 44, and the access device 32 have been patterned andetched to form an individual memory cell 20, a layer of insulativematerial 62, such as silicon nitride, is deposited over the structure,as illustrated in FIG. 14. A layer of oxide 64 is then deposited overthe insulative layer 62. Finally, the oxide layer 64 is patterned and acontact hole 66 is formed through the oxide layer 64 and the insulativelayer 62, as illustrated in FIG. 15. The contact hole 66 is filled witha conductive material to form the word line 22.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. A method of fabricating a memory cell, said method comprising the steps of: (a) forming an access device on a semiconductor substrate; (b) depositing a layer of dielectric material on said access device; (c) forming a pore in said layer of dielectric material, said pore being smaller than a photolithographic limit; (d) depositing a first layer of conductive material within said pore to form a first electrode; (e) depositing a layer of structure changing material on said first electrode; and (f) depositing a second layer of conductive material on said layer of structure changing material to form a second electrode.
 2. The method, as set forth in claim 1, wherein step (a) comprises the step of forming a diode.
 3. The method, as set forth in claim 2, wherein the step of forming a diode comprises the steps of: forming a layer of N doped polysilicon disposed on said substrate; and forming a layer of P+ doped polysilicon disposed on said N doped layer of polysilicon.
 4. The method, as set forth in claim 1, wherein step (c) comprises the steps of: forming a mask over said layer of dielectric material, said mask having a window therein exposing a portion of said layer of dielectric material, said window being sized at said photolithographic limit; forming a spacer within said window, said spacer covering a peripheral portion of said exposed portion of said layer of dielectric material to create a second window exposing a portion of said layer of dielectric material smaller than said photolithographic limit; and removing said exposed portion of said layer of dielectric material created by said second window to create said pore.
 5. The method, as set forth in claim 1, wherein step (d) comprises the steps of: depositing said first layer of conductive material onto said layer of dielectric material and into said pore using collimated physical vapor deposition; and removing portions of said first layer of conductive material deposited onto said layer of dielectric material.
 6. The method, as set forth in claim 1, wherein step (e) comprises the step of: depositing a layer of chalcogenide material on said first electrode.
 7. The method, as set forth in claim 1, wherein step (e) comprises the step of: depositing said layer of structure changing material in said pore.
 8. The method, as set forth in claim 1, further comprising the step of: forming a layer of insulative material substantially surrounding said access device, said layer of dielectric material, said layer of structure changing material, and said second electrode.
 9. A method of fabricating a memory array, said method comprising the steps of: (a) forming a plurality of access devices on a semiconductor substrate; (b) forming a first plurality of conductive lines, each of said first plurality of conductive lines being coupled to respective access devices; (c) depositing a layer of dielectric material on said access devices; (d) forming a plurality of pores in said layer of dielectric material, each of said pores being smaller than a photolithographic limit; (e) depositing a first layer of conductive material within each of said pores to form a plurality of first electrodes; (f) depositing a layer of structure changing material on each of said first electrodes; (g) depositing a second layer of conductive material on said layer of structure changing material to form a plurality of second electrodes; and (h) forming a second plurality of conductive lines, each of said second plurality of conductive lines being coupled to respective second electrodes.
 10. The method, as set forth in claim 9, wherein step (a) comprises the step of forming a plurality of diodes.
 11. The method, as set forth in claim 10, wherein the step of forming said plurality of diodes comprises the steps of: forming a layer of N doped polysilicon disposed on said substrate; and forming a layer of P+ doped polysilicon disposed on said N doped layer of polysilicon.
 12. The method, as set forth in claim 9, wherein step (d) comprises the steps of: forming a mask over said layer of dielectric material, said mask having a plurality of windows therein exposing portions of said layer of dielectric material, said windows being sized at said photolithographic limit; forming a spacer within each of said windows, each spacer covering a peripheral portion of said respective exposed portions of said layer of dielectric material to create a second window exposing a portion of said layer of dielectric material smaller than said photolithographic limit; and removing said exposed portions of said layer of dielectric material created by said second windows to create said pores.
 13. The method, as set forth in claim 9, wherein step (e) comprises the steps of: depositing said first layer of conductive material onto said layer of dielectric material and into said pores using collimated physical vapor deposition; and removing portions of said first layer of conductive material deposited onto said layer of dielectric material.
 14. The method, as set forth in claim 9, wherein step (f) comprises the step of: depositing a layer of chalcogenide material on said first electrodes.
 15. The method, as set forth in claim 9, wherein step (f) comprises the step of: depositing said layer of structure changing material in said pores.
 16. A method of fabricating an array of pores, said method comprising the steps of: forming a mask over a layer of dielectric material, said mask having a plurality of windows therein exposing portions of said layer of dielectric material, said windows being sized at said photolithographic limit; forming a spacer within each of said windows, each spacer covering a peripheral portion of said respective exposed portions of said layer of dielectric material to create a second window exposing a portion of said layer of dielectric material smaller than said photolithographic limit; and removing said exposed portions of said layer of dielectric material created by said second windows to create said pores. 